A typical telephone, in particular an industrial telephone, has a handset, a housing, and a display.
Telephones of this type generally serve as means of communication for transmitting sound, and particularly voices by means of electric signals. Industrial telephones are further characterized by a particularly robust construction and are advantageous for use in, for example, wet or dusty environments, as well as generally under any harsh conditions.
Industrial telephones as used in practice are often equipped with a closed housing having a door that when closed protects the handset. Since the handset is inside the housing in order to protect the entire unit from unfavorable environmental conditions, such as in a tunnel, within a chemical plant, or railway track system, etc., additional measures are required in order to be able to signal an incoming call. To this end a relay has mostly been previously hooked into the line connected to the telephone so as to actuate a for example external lamp or loudspeaker to signal an incoming call. In this manner any personnel near the telephone, such as security personnel, may be optically and/or acoustically informed of an incoming call. Such measures are extensive, because an external lamp or loudspeaker is required, and a relay or another switching element must be connected to the line.
According to the prior art of DE 203 08 060 a portable telephone has a display that incorporates signaling means. Such signaling or illumination means may be a plurality of light-emitting diodes in the housing. Furthermore, a light-conducting element covering the light-emitting diodes is discussed. The light-emitting diodes may be made to blink by a controller to indicate an incoming call. This represents a modest improvement in the signaling of telephone-specific functions of the telephone.
A further system as described in DE 86 06 944 is a car phone equipped with a light source in the handset. In this manner the alerting may be recognized from the exterior.
In DE 87 15 481 a phone is described having a transparent housing. A light rod is provided inside the housing. As soon as a call is received, the pulsing of associated light sources activates and deactivates in order to create a blinking light.
U.S. Pat. No. 6,711,257 describes a telephone where an indicator is incorporated into the handset. It may be an LED in combination with a light conductor.
In DE 86 06 944 a light source is provided inside the handset underneath a lens for signaling purposes.
Finally, DE 35 13 153 describes a telephone that is equipped with a light bulb. The light bulb irradiates a luminescent plate which illuminates the entire background of the display after the operating receiver has been lifted.
The prior-art telephones have some problems. For example, one disadvantage with current industrial telephones is that the optical signal is an extra part and is somewhat complex and expensive. Furthermore, if any signal displays according to prior art are used, it becomes obvious that they are predominantly used in cell phones and are therefore integrated into the handset. Furthermore, solutions using an entire illuminated housing are not suited for industrial applications where the housing can get so dirty that it becomes opaque.
Furthermore, the cell phones described in much of the prior art do not work, for example, in tunnels, on the ocean, etc., and are therefore also not suitable in many industrial applications as an alternative to standard hard-wired telephones. Hence the known measures used in cell telephones are not suitable for such hard-wired telephone systems with a closed housing.